1. Field of the Invention
The present invention relates to an apparatus for polishing semiconductor wafers, and more specifically to an apparatus for mirror polishing the surfaces of the semiconductor wafers.
2. Description of Related Art
In order to increase the yield in the device process, a demand for the flatness of semiconductor wafers keeps increasing these days. For satisfying the demand for the high-flatness, the process technology before polishing has been improved whereby it has become possible to obtain semiconductor wafers having a uniform thickness. However, there is a drawback that a convex-surface effect occurs as being polished in the polishing step performed subsequently, which destroys the high-flatness. It -has been already known that the convex-surface effect acceleratedly increases when a duration of the polishing time exceeds a certain period of time.
Therefore, in order to maintain the high-flatness, there is provided a multi-step polishing method in which, the polishing is carried out until the convex-surface effect begins to increase and, after a certain rest time has elapsed, the polishing is carried out again; this procedure is repeated again and again, whereby semiconductor wafers with lesser convex-surface effect can be obtained.
To realize this multi-step polishing method, as shown in FIG. 4, there is a method of polishing wafers one by one with the use of conventional single wafer polishing apparatus, in which conventional single wafer polishing apparatus 4a-4c are disposed and transfer apparatus 5a-5b are arranged between the conventional single-wafer polish apparatus 4a-4c. That is, a semiconductor wafer 10 is polished, step-by-step, by each of the single-wafer polish apparatus as it is transferred between the polish apparatus by the transfer apparatus 5a and 5b.
However, in the case where the multi-step polishing is carried out while processing the wafers one by one with the use of the conventional single-wafer polishing apparatus, it is required that the number of polish apparatus and the number of transfer apparatus must be equal to the number of steps of the multi-step polishing, hence there is a problem that a space required for the polishing becomes too large.
Moreover, because attaching and detaching to and from top rings must be repeated while moving between the plurality of polishing apparatus and transfer apparatus, there has been a problem that spots and scratches tend to occur.